1. Field of the Invention
This invention relates to a novel process for the preparation of certain improved hydrocarbon conversion catalysts. More particularly, the invention relates to the use of crystalline aluminosilicate zeolite-containing catalysts which have been utilized in hydrocarbon conversion reactions and which have lost at least a portion of their initial activity as a starting material for the preparation of such improved hydrocarbon conversion catalysts.
Still more particularly, the present invention relates to the re-use of crystalline aluminosilicate zeolite hydrocracking catalysts which have been rejuvenated.
2. Description of the Prior Art
The catalytic treatment of hydrocarbons, particularly those derived from petroleum feed stocks, has seen great improvement in the past several years. For example, the early clay catalysts were replaced by synthetic, amorphous, alumina or silica-alumina composite catalysts which afforded significant improvements in activity, selectivity, stability and attrition resistance. More recently, however, even further improvements have been made with the introduction of crystalline aluminosilicate zeolites which exhibit very much greater activity and selectivity toward hydrocarbon conversion reactions than the amorphous silica-alumina type catalysts.
Crystalline aluminosilicate zeolites, which are commonly referred to as "molecular sieves", are now well known in the art. They are characterized by their highly ordered crystalline structure and uniformly dimensioned pores, and are distinguishable from each other on the basis of composition, crystalline structure, adsorption properties and the like. The term "molecular sieves" is derived from the ability of these zeolite materials to selectively adsorb molecules on the basis of their size and form. Various types of molecular sieves may be classified according to the size of the molecules which will be rejected (i.e. not adsorbed) by a particular sieve. A number of these zeolite materials are described, for example, in U.S. Pat. Nos. 3,013,982 - 3,013,986, wherein they are characterized by their compositions and X-ray diffraction characteristics. In addition to their extensive use as adsorbents for hydrocarbon separation processes and the like, it has recently been found that crystalline aluminosilicate zeolites, particularly after cation exchange to reduce their initial alkali metal oxide content, are valuable catalytic material for various processes, particularly hydrocarbon conversion processes. The ion exchange of these crystalline aluminosilicate zeolites with various metals and metal ions is described, for example, in U.S. Pat. No. 2,971,904 and U.S. Pat. No. Re. 26,188.
In general, the chemical formula of the anhydrous form of the crystalline aluminosilicate zeolite, expressed in terms of mole ratios of oxides, may be represented as: EQU 0.9 .+-. 0.2 M.sub.2/ n O:Al.sub.2 O.sub.3 :X SiO.sub.2
wherein "M" is selected from the group consisting of hydrogen, monovalent, divalent, and trivalent metal cations and mixtures thereof; "n" is its valence, and "X" is a number from about 1.5 to about 12, said value dependent upon the particular type of zeolite. The zeolite, as synthetically produced or found naturally, normally contains an alkali metal such as sodium or an alkaline earth metal such as calcium. Among the well-known naturally-occurring zeolites are mordenite, faujasite, chabazite, gmelinite, analcine, erionite, etc. Such zeolites differ in structure, composition, and particularly in the ratio of silica to alumina contained in the crystal lattice structure. Similarly, the various types of synthetic crystalline zeolites, e.g. synthetic faujasite, mordenite, erionite, etc. will also have varying silica-to-alumina ratios depending upon such variables as the composition of the crystallization mixture, reaction conditions, etc. Silica-to-alumina ratios higher than 12, e.g. as high as 90 to 100, can also be achieved through various methods for the removal of alumina from the crystal structure of the zeolite. Such zeolites having these higher silica-to-alumina mole ratios are preferred in hydrocarbon conversion processes because of their high stability at elevated temperatures. Those silica-to-alumina ratios above three are particularly preferred.
The processes for producing such zeolites synthetically are now well known in the art. The crystalline zeolites are prepared by having present in the reaction mixture: Al.sub.2 O.sub.3 as sodium aluminate, alumina sol and the like; SiO.sub.2 as sodium silicate and/or silica gel and/or silica sol; an alkaline hydroxide, e.g., sodium hydroxide, either free or in combination with the above components. Careful control is kept over the alkali concentration of the mixture, as well as the proportions of silica to alumina and soda (metal oxide) to silica, the crystallization period, etc., all in a manner known per se. A general scheme for preparing large pore size (i.e. 6-15A) crystalline aluminosilicate zeolite would be as follows:
Colloidal silica, such as commercial Ludox, is mixed with a solution of sodium hydroxide and sodium aluminate at ambient temperatures. The reaction mixture may be allowed to digest at ambient temperatures for periods of up to 40 hours or more, e.g. 24 hours. The reaction mixture is then heated to 180.degree.-250.degree. F., preferably 200.degree.-220.degree. F., for a period of 24 to 200 hours or more, in order to effect crystallization. The crystalline aluminosilicate is then separated from the aqueous mother liquor by decantation and washed and thus recovered as a crystalline product having a particle size of about 0.05 to 5 microns. Synthetically prepared alumino-silicate zeolites having large effective pore diameters have been termed in the industry as Zeolites "X", "Y", "L", etc.
For use in many commercial operations, e.g. moving or fluidized bed, adsorption or hydrocarbon conversion processes, the difficulty of handling the extremely fine size zeolite crystals has required the use of various "matrices" in conjunction with the zeolites. The use of such matrices is described in U.S. Pat. Nos. 3,140,249 and 3,140,253. The matrix may include various organic and/or inorganic compositions, particularly inorganic oxide gel compositions, including plural gel compositions containing a major amount of silica in conjunction with one or more metal dioxides selected from Groups I-B through VIII of the Periodic Table, particularly such compounds as alumina, magnesia, zirconia, titania, etc. The matrix may also include such compositions as clay materials, particularly kaolin-type clays.
Recently such crystalline aluminosilicate zeolite-containing hydrocarbon conversion catalysts of greatly improved stability have been discovered which contain reduced amounts of alkali metal, such as less than 3%, and preferably less than 1%, e.g. see U.S. Pat. No. 3,449,070.
Typically, these crystalline aluminosilicate zeolite-containing catalysts are utilized in hydrocarbon conversion reactors at various temperatures, generally ranging from about 500.degree. to 800.degree. F. depending on the particular reaction desired, and utilizing various hydrocarbon-containing feedstreams, such as light virgin naphtha, heavy gas oil, catalytic cycle stock, heavy reformate, coker gas oil, etc. These reactors are generally at pressures of from 500 to 2200 psig. During use in these reactors, these zeolite-containing catalysts tend to become deactivated, and are generally cyclically regenerated by high-temperature treatment with an oxygen-containing gas in order to burn off accumulated carbon and to restore some of their lost activity. After such continued use, however, these catalysts have tended to exhibit increasingly lower activities, and eventually must be discarded and replaced by fresh catalyst.